Heretofore various circuits have been proposed for measuring alternating current. These previous circuits measured various parameters such as zero crossings and maximum amplitude of alternating current waveforms. Some of these circuits included sample and hold circuits, comparators, pulse shapers, ramp generators and/or other wave sensing and analyzing circuitry. The data acquired was processed with analog and/or digital computer circuitry.
Examples of such previously proposed circuits for measuring parameters of an alternating waveform, voltage or current, are disclosed in the following U.S. Patents.
______________________________________ U.S. Pat. No. PATENTEE ______________________________________ 3,663,956 Purdy et al. 3,731,152 Rockefeller, Jr. 3,758,763 Nohara, et al. 3,944,935 Plant 4,051,470 Esteban, et al. 4,069,452 Conway, et al. 4,073,008 Andow et al. 4,073,009 Andow, et al. 4,074,190 Bunting 4,283,713 Philipp ______________________________________
As will be described in greater detail hereinafter the method and apparatus of the present invention for measuring alternating current differ from the previously proposed circuits for measuring parameters of a waveform described in the patents identified above by providing digital circuitry for sensing at least two data points on an alternating current waveform, such data points being the crossing by the waveform of a negative reference voltage value and the crossing by the waveform of a positive reference voltage value, the absolute value of the negative reference voltage being equal to the absolute value of the positive reference voltage. The method and apparatus correlate fixed data points or amplitude values of the waveform with measured time intervals between the crossings by the waveform of the fixed amplitude values to estimate and measure parameters of the waveform such as frequency, amplitude and DC offset.
Moreover, the apparatus of the present invention differs from the previously proposed circuits for measuring parameters of a waveform by utilizing digital processing circuitry only and does not use an A/D converter.
Also, for the purpose of measuring frequency and DC offset, the method and apparatus of the present invention sense five reference voltage crossings. By measuring these five reference voltage crossings and the time intervals between these crossings, the method can be utilized in making a good estimation of the amplitude of the fundamental as well as the frequency and any DC offset of the waveform.
As will be described in greater detail hereinafter the method of the present invention as well as the apparatus for practicing the method encompasses measuring or noting the times at which the instantaneous value of an alternating waveform assumes (passes through or crosses) one or the other of two predetermined reference values. This differs from existing practice utilizing analog to digital converters where the measurements of the instantaneous values of the waveform are made at different prescribed times.
The method of the present invention is particularly adapted for digital measuring of an alternating waveform where it is desired to process the acquired waveform data or information in a microprocessor or other digital data processing equipment and provide advantages over systems which use an A/D converter and sample and hold circuitry. In this respect, A/D converters have high percentage quantization errors at low signal levels and unnecessary precision at high signal levels. In contradistinction with the method and apparatus of the present invention, there is high resolution of low level,signals and decreasing resolution as the signal level increases. These resolution characteristics are highly desired in overcurrent relaying.
Moreover, the apparatus of the present invention has fewer and less costly components, i.e., a comparator and a few logic gates, than are used in an A/D converter system, i.e., an expensive A/D converter and sample and hold circuitry. Additionally, the apparatus of the present invention has a much faster response time than conventional A/D converter systems.